Purification of isoprene



m FF- n' We 2,935,540 I P U' RIF IC'ATIGN 6F "ISGPRENE Johns. in. Wolfe,Rocky River, Ohio, assignor to- GOOdrich-Gulf Chemicals, Inc.,Cleveland, Ohio, a corpora tion of Delaware No Drawing. ApplicationApril 22,

Serial-No. 654,063

'z Claims. (onto-681.5

This invention relates to the purification of. isoprene; Moreparticularly,. it is concerned with the-purification ofconnnercially-available crude isoprenet'o produce prod ucts of asufiiciently-high order ofpurity as tobe polymerizable in good .yieldsby the" new, structurally-specific types of catalysts.

In the copending application ofSamueLE. Horne, In, Serial No. 47 2,786filed December 2, 1954, there is disclosed a heavy-metal organo-metalliccatalyst'for converting isoprene to an all-cis-1=,4 (head-to-tail)polyisoprene resembling natural (Hevea) rubber in structure and in manyof its properties. -Likewise, in the copending, application of C. F.Gibbs et -al., Serial-No.;503,0-2 7, filed April 21 1 955, theproduction of an essentially all-trans- 1,4 head-to-tail polyisopreneresembling bala-ta isdescribed using. catalystssimilar tothat of theabove-men- 2,935,540 Patented May 3, 1960 tion of the other acetylenichydrocarbon inhibitors or other inhibitory impurities. The molecularsieves have to be regenerated when they have become saturated, and thisoccurseffectively when the sieves have absorbed about to of their weightof 2 butyne. While the sieves can be regenerated, such a process step iscostly- Moretioned' I-Iorneapplicationr -Furthenin the copending application of Hugh E. Diem et al.,.-Serial No. 557,826,

filed January 9 1956, now US. Patent No. 2,913,444,

there is disclosed an alkyl'lithium catalystior producing as high'cis.-1 -,4 ,polyisoprene. In any of these processes; commercirllyavailable isoprenes function as if they contain varying amounts ofinhibitors or retarders of polymerization.

Crude or commercial isoprenes obtained from hydrocarbon conversionprocesses, for example, the cracking or dehydrogenation of hydrocarbons,are found to contain as inhibiting substances, straight chain acetylenessuch as Z-butyne and l-pentyne, and alpha-acetylenes such as isopropylacetylene and isopropenyl acetylene. Some commercial is'oprenesalsocontaincyclopentadiene '1,3 which, strangely enough, is among? the =mostpotent inhibitors of "the polymerization of isoprene. Of theseimpurities or inhibitors, Z-butyneseems to be present-in the largestproportion, often being, present to the-extent of as much as 3 or 4 molpercent. The alpha-acetylenes may be present in proportions up toabout'1 wt. percent and cyclopentadiene-1,3 up to about 0.5 to 0.7 wt. .per-

cent.

There have been many methods proposed for treating isoprene and similarmonomers to remove the above and other impurities.- For example,isoprene can be treated with maleic anhydride to removecycl'opentadiene-lfi. It has also been proposed to treat isoprene withmetallic sodium to eiiect a reduction intotal inhibitors. However, nosingle treatment of this "type-nor any combination of these treatments,has been found capable of producing an Angstrom units. The finelydivided metallic sodium reacts quite readily with a-number of inhibitorysubstances likely to be present in isoprene, particularly with thealpha-acetylenessuch asisopr-open-yl acetylene. While'it I has beenfound that authentic (pure) Z-butynedoes-not react with metallic sodium,an efficient treatmentv of i prene with finely-divided metallic sodiumgenerally seems to eiiect an appreciable reduction in the apparent 2-butyne content. The term apparent Z-butyne content? is used herein. todesignate that portion of-a crudemiso prene whichshows up asthe Z-butynepeak uponana lysis with the Fisher-Gulf .partitioner employing atricresyl phosphate column -at' 50 C.- Apparently, the partitioner peakcovers at -leas t two compounds, one of which isZ-butyenetwhich is not:reaetive withtmetallic sodium) andthe other, an unknownsubstance, whichis reactive with-metallicsodium. The treatment with,fine1y divided. ordispersed sodium also effects a reduction in other inhibitory impuritiessuch as cyclopentadienepoxygen present in peroxides; and in the amountof water, phenols and phenolic inhibitors, alcohols, and the 'like. Thetwo-st-age process of this invention produces isoprene of a high degreeof pol-ymerizability. By this procedure the totaliinhibitors ofstereo-specific polymerizationcan be reduced to a-relatively harmlesslevel (-i.e. less; than about 0..l:%-by Weight). I

The sodium treatmentindicated above is conductedv in such a. manner asto prevent anysubstantial amountsof polymer formation (sodium normallyis a polymerization catalyst for isoprene). The latter is accomplishedin any of a number of ways including (1) shortv contact time; (2)reducedtconccntration-of sodium metal; and (3) keeping thetemperature-below about 30 to 40 CnWhen the sodiumconcentration andtimeof. contact would otherwise 'favor polymerization. It is possible tobubble-isoprene vapor through molten sodium metal at about Crwithoutsubstantial polymerization because of thee);- tremely short-contacttime. it ispreterredto utilize only sufiicientmetallic-sodiumto insurereaction with the full amount, of sodium-reactive-impurities, any excessbeing limited (in the-usual case. of appreciable contact. times) tonotmore than about a 75% excess over the theoretical sodium requirement.Toturther facilitate-theireaction and drive the reaction to completion,the sodium should be as finely divided as possible. Preferred isafinesodium metal dispersion Which'is colloidal in nature, the.-continuousphase being a hydrocarbon such-aspetrolatum or petrolaturn mineral oilmixtures. The latter are made by melting sodium and the petrolatum andcombining (under an inert atmosphere such as helium) the resulting meltsunder conditions of vigorous agitation. Another form of sodium which canbe utilized is-rnade byecoating dry Na COr with molten sodium andthencoolingr. Such products should be stored under helium to preventreaction with oxygen and moisture of the air.

It is necessary to carry out the sodium treatment under an inertatmosphere, not only to prevent reaction of moisture and oxygen with thehigh-reactive sodium but also to prevent pick-up of these impurities bythe isoprene. The time of contact between the sodium metal and theisoprene is not critical, only that time being required as is necessaryto accomplish efiicient and thorough contact of the sodium with theentire body of isoprene. Usually, a contact time of one or two hours isentirely adequate in batch-style reactions.

Following the mingling with the-sodium, the isoprene is preferablyfiltered, centrifuged, and/or distilled to accomplish a clean separationbetween the isoprene, on the one hand, and the unreacted sodium metaland the reaction products, on the other hand. Once freed of the sodiummetal and its reaction products, the treated isoprene is ready forremoval of the 2-butyne type ingredient, the second stage sievetreatment.

However, at this point it has been found highly desirable to subject theisoprene to a distillation wherein only about to about of the isopreneis taken off overhead under reflux, the overhead being found to berelatively richer in 2-butyne than the pot material. The condensate canbe recycled or treated by other methods to separate the isoprene fromthe Z-butyne. In some cases, the condensate may contain up to 50% of theZ-butyne content of the sodium-treated isoprene. The total reduction inthe over-all apparent Z-butyne content can be as much as 75% and theuseful life of the sieves will be more than doubled. When sodium-treatedand distilled isoprene is treated with sieves, and then stored under adry, inert atmosphere, it will be found to contain less than 0.1% byweight of inhibitory substances.

As is known in the art, molecular sieves are crystalline dehydratedzeolites, natural or synthetic, having a well-defined physicalstructure. Chemically the zeolites are hydrous aluminum silicates,generally containing one or more sodium, potassium, calcium, strontiumor bariurn cations, although zeolites containing hydrogen, ammonium orother metal cations are also known. These zeolites have a characteristicthree-dimensional aluminum silicate anionic network, the cationsneutralizing the anionic charge. Upon dehydration, the three-dimensionallattice network of the crystal is maintained, leaving interconnectingchannels, pores, or interstices of molecular dimensions within thecrystal lattice. The cross-sectional diameter of such channels can vary,dehydrated three-dimensional zeolites having channels withcross-sectional diameter of 4, 5 or 6 Angstrom units being known.

For each of the zeolites of this type, the narrowest cross-sectionaldiameter of the channels is characteristic and is substantially uniformand fixed throughout the crystal. Thus, materials are available havingchannel diameters of substantially all 4 Angstrom units, substantiallyall 5 Angstrom units, or substantially all 6 Angstrom units, as the casemay be. It is customary, therefore, to characterize certain molecularsieves found useful in this invention as 5 Angstrom unit molecularsieves. It is a characteristic feature of this invention that theselective absorption of straight-chain acetylenes from isoprenecontaining them takes place only with 5 Angstrom molecular sieves. Thus,Z-butyne, for example, cannot be separated from isoprene when usingeither 4 Angstrom or 6 Angstrom molecular sieves.

The contact between the sodium-treated (or sodiumtreated/ distilledisoprene) and the 5 Angstrom sieves can be effected in many ways. Forexample, the isoprene can be in either the liquid or vapor state whencontacted with the molecular sieves. In either case, the straightchainacetylenic inhibitors, and particularly Z-butyne, are efiectivelyremoved. Since the removal of these acetylcnic inhibitors is anadsorption process, the process takes place under normal ambienttemperatures and pressures, particularly when the isoprene is in theliquid form. However, temperatures ranging from about 20 to about 150 F.and pressures ranging from about 1 to about 250 pounds per square inchabsolute can be employed. Higher temperatures and pressures than thosegiven are not preferred because of the possibility that the isoprene maypolymerize. Isoprene has a boiling point of 93.4 F. (34.1 C.) and it ispossible that even at room temperatures there may be isoprene vaporpresent when conducting a' liquid phase contact process.Superatmospheric pressures of, for example, up to about 250 pounds persquare inch absolute, can be employed to prevent vaporization atordinary temperatures. Where an entirely vapor phase contact is desired,the contact temperature can be increased to about F. or the pressureobtaining can be reduced to induce vaporization of the impure isopreneat temperatures below its, normal boiling point.

The actual physical particle size (i.e. the state of subdivision) of themolecular sieves is not critical. Successful results can be obtainedwith molecular sieve powders having an average particle size of from 0.5to 5 microns, as well as with pellets having diameters of A; and inch.Thus, in small operations, isoprene can be stirred with the powderedtype of 5 Angstrom molecular sieves, for example, in the proportions offrom about 200 to about 400 grams of molecular sieves per gram mol ofstraight-chain acetylene (2-butyne) content, the mixture allowed tosettle and an isoprene substantially free of Z-butyne decanted off. Forlarger scale operations, wherein it is the intention to reclaim orregenerate the sieves, it is preferred to employ the pellet-stylemolecular sieves in an adsorption column style of operation, thepellet-style sieves being more convenient to handle and assuring properflow of isoprene through beds of the sieves.

For example, a column or vessel packed with a fixed bed or layer ofsieves can be utilized and the isoprene passed through the fixed bed ineither upflow or downflow direction. Likewise, moving beds of the sievescan be utilized. In the latter method, isoprene is passed upwardlythrough a downwardly moving bed of sieves;

spent or saturated sieves are removed from the bottom of the bed,regenerated and recycled to the top of the downwardly moving bed, andpurified isoprene is removed from the top of the adsorption zone. Fixedbed operation can be made more or less continuous in nature by providingtwo or more beds of sieves, one of which is on stream while another isbeing regenerated.

While the contact time, per se, is not critical, in fiowtype processesof either fixed or moving bed types, certain contact times usually arerequired and these will vary depending upon such factors as the type ofcontact (i.e. liquid or vapor phase contact) and the like. In suchprocesses the contact time can be expressed most conveniently as liquidhourly space velocities, values of which from about 0.3 to 20 volumes ofimpure isoprene per volume of molecular sieves per hour can be employed.With fixed bed operation, space velocities of from about 0.3 to 8 arepreferred.

The total through-put of impure isoprene before the molecular sievesbecome spent will vary considerably, depending principally on the2-butyne content of the isoprene. In general, when the molecular sieveshave absorbed from about 6 to 9 or 10% by weight of acetylenicmaterials, their absorptive capacity is markedly reduced and they may beconsidered saturated. When this occurs, the sieves may be regenerated byburning oil, under closely controlled conditions, the adsorbedhydrocarbons with an oxygen-containing gas at a temperature controlledrather closely in the range of about 900 to 1100 F. Regeneration bydesorption techniques such as hot purge gases, steaming, evacuation, orotherwise employing elevated temperatures involves the risk of foulingof the sieves due to the polymerization of the 'adsorbedun saturatedhydrocarbons. I 7

Regeneration of molecular sieves is bestac'compli'shed by first drainingon of liquid isoprene-and'then' evacuau ing. the container to draw offas much residualisopre'rte as possible. A mixture of gassuchasfainornitrogeh (or combustion gases) with a carefully controlledamount of oxygen is preheated andmenpassed into the sieves, thegasmixture being initially atle'ast, preheated about 900 F. do notcompletelyremove'the adsorbed hydrocarbons. Good-results-areobtainedinthe range of 900 'to 950" F. After the burned,the'sievesare'purged of residual combustion gases and-finally cooled-toroom temperature while carefully protected by a dry gas such asnitrogen. The sieves canthenbe utilized again for removal of acetylenicimpurities such as Z-butyne. The sieves retain their efiiciency evenafter repeated cycles of regeneration.

Since molecular sieves'adsorb Water in preference to all othersubstances, thereby -impairing their capacity for acetylenic materials,it is desirable 'to guard against the adsorption of water. Tothis-end,the sieves are handled when charging to the apparatus) under a dryatmosphere and, to insure their completely anhydrous condition, they maybe preheated to about 250 to 650 F. just-prior t'o contact with theisoprene in the adsorption step. Also,

to avoidpremature deactivation of the molecularsieves, it is desirable,if theiso'prene has been exposed to moistnre, to dehydrate the isopreneprior to'contactin'g the 5 Angstrom molecular sieves. For the latterpurpose, 'activated alumina, silica gel, ormolecu'lar sieves (anychannel diameter) can be utilized. However, if the isoprene is protectedby a dry gas atmosphere after the first stagesodium treatment, theisoprene will be in a properly 'anhydrous condition for contact withthe'sieves. In the regeneration of the sieves, it is best t'ov purgethe" freshly burnt-elf sieves of residual combustion gases beforethesieves are cooled, thereby to guard against condensation of any water ofcombustion in the sieves. Pickup of moisture can be guarded againstduring cooling or while onstand-by by maintaining an atmosphere of drygas over the sieves. If a stream of cooling gas is employed afterregeneration, the gasshould be desiccated before contact with thesieves.

Of course, to protect the purity of the sodiumand sieve-treatedisopren'e, it should be stored under an inert atmosphere to preventpickup of moisture and oxygen during storage. In any case, the isoprenewill be sufiiciently pure, if justbefore use in polymerization, it istopped to the extent of about 5%. Such a procedure will ensurepolymerization at maximum rates. The small amount of overhead materialcan be recycled to either stage of the above purification process.

The invention'will now be described in connection With several specificexamples, which are intended as being illustrative only.

Example I In this and in Example 11,. the starting isoprene utilizedis apetroleum-derived 95% commercial product having the following inhibitoranalysis:

Alpha-acetylenes 0.31 wt. percent. Cyclopentadiene 029 wt. percent.2-butyne 3.58 mol percent. Tertiary butyl catechol 0.05 wt. percent.

1 An inhibitor added by the manufacturer,

'Z-butyne mined by titration with alcoholic AgNO o1ntion; "thecyclopentadien'e is determined colorimetricallytand'the' ployed. Thedispersion containsabout 2 5.6 byweiglit' "(if 'SOdilllh' ii'l 'thefOfmOf P'Ei'ItiCles which range fldmdtb 10 microns in diameter. Such adispersion is made by. melting petrolatum, adding mineral oil andlinseed oil dispersant and then combining the resultant-melt with meltedsodium under a dry nitrogen-atmosphere while vigorously agitating themix. The materials utilized are as follows:

Kaydol petrolatum e lbs; 6 .Fonoline white mineral oil... "lbs"- 9Boiled linseed oil (di'persant) "grains" 23 Sodium metal "lbs; '5

In this example, a 12 liter glass laboratoryfiask, is washed, dried andpurged with dr-y nitrogen. A. stirrer and a 6-pronged sintered-glassfiltering rodare provided as accessories. Into the nitrogen-filled flaskthere are charged 4875 grams of the above des'c'rib'ed raw iseprene; Inthis experiment it is decided to emptoy an excess of sodium metal ofabout 40% over thatr'equire'd,"theoreti cally, to reactwiththeabove-indicated'amount. otalphaacetylene and cyclopenta'diene.Accordingly, 3-1.5 grams of metallic sodium are added tothe fi'ask (Ca.l26?grams of the above described sodium dispersiorr). Therontent's ofthe flask, under a nitrogen atmosphere, then is agitated gently at roomtemperature (25 C.) for'tx'vohour's. To follow the course of thereaction, samples of the isopren'e aretakenfor analysis-$0.5, -1.-5'an'd at -2L0hours The isoprene is then drawn off through the filter:rod intoa'nother dry, nitrogen-filled flask for the sieve-treatment;

Analysis of the samples shows the following:

7 Percent Sample Alpha- 7 Acetylene -0.5 hr wt. percent- 0. 15 rain do0036 2.0 hr. d0- 0.025 2-Butyne Content (Fmal). mol percent 2. 862-Butyne Content (Initial)- I do. 3. 58 Cyclopentadiene (Final). wt.percent 0. 0

As will be seen the above sodium treatmentfreduced the alpha-acetylenecontent to 0.025 wt; "percent. Urea: pectedly, the Z-butyne content isreduced from 3.58 mol percent to 2.86 mol percent.

The resultant sodium-treated isoprene', in" amount 4218- grams, iscombined with 844 grams ofpowdered; anhydrous 5 Angstrom molecularsieves 3'tl4grams ofr sieves/mol of 2-butyne to be removed). The-nitrogen flow over the mixture is maintained for 2 hours while gentlyagitating. During this time the reaction mixture is at 26 C. After 2hours of agitation the isop'rene isfiltered ofi (through a filter rod)into a'dry, nitrogen-filled still pot. The product is then flashdistilled; Analysis of the flash distilled product reveals thefollowing:

Alpha-acetylenes 0.025 wt. percent.

0.l mol percent. Cyclopentadiene 0.

The flash-distilled product is tested for-its" reaction rate in astandard polymerization in which a pure grade isoprenetPhillipsPetroleum Co. pure grade isoprene) and the original impure isoprene areemployed as references or controls. Each polymerization is carried outin acnequart glass beverage bottle, the bottle beingsealed and clampedin a rack rotating in a water bath. Each bottle is charged under a flowof dry nitrogen with about 40 grams of one of the above isoprenes, about500mlof Percent Wt. Yield 95100% in 15 hrs.

(2) Purified Isoprenc of Example I 1(0% in 15 hrs.

(3) "Pure Grade" Isoprene Example II In this example, the procedure ofExample I is repeated except that the sodium charge factor is 1.67 (i.e.a 67% excess) instead of 1.40. In spite of the increased proportion ofsodium no polymerization is noted. The isoprene, following 2 hours ofreaction with the sodium dispersion at 25 C. has the following analysis:

Alpha-acetylene 0.006 wt. percent. 2-butyne 2.85 mol percent.Cyclopentadiene None.

The increased proportion of sodium has reduced the alpha-acetylenecontent below 0.01%. Again, it should be noted that the 2-butyne contenthas been reduced 'to 2.85 mol percent from an original value of 3.58 molpercent. 1

After the above sodium-treated isoprene is treated with 5 Angstrommolecular sieves, according to the procedure of Example I, the analysisof the isoprene is as follows:

Alpha-acetylene; 0.006 wt. percent. Z-butyne 0.1 mol percent.Cyclopentadiene None.

Total acetylenic inhibitors Less than 0.10 wt. percent.

The resulting isoprene is stabilized by addition of 0.05 wt. percent oftertiary butyl catechol and stored under dry nitrogen until required forpolymerization. This iso prene polymerizes with even greater facilitythan does the isoprene of Example I.

Example III In this example, a larger quantity of isoprene is puri: fiedby a process consisting of the steps: 1) sodium treatment using thesodium dispersion of Example I and (2) treatment with 5 Angstrommolecular sieves in a packed column (fixed bed). In this experiment thereactor utilized in the sodium treatment is a 1000 gallon reactorequipped with a distillation column. First, the pot is evacuated whileheating same at 30 C. for 2 hours. After cooling to room temperaturewhile filled with dry nitrogen (15 p.s.i. gage), 3,486 lbs. of impureisoprene is pumped in. To this charge there are added 10.61 lbs. ofmetallic sodium (added in the form of the same dispersion used inExample I). The resulting mixture is gently agitated for one hour and 40minutes at 23-25 C. The charge is then filtered and samples withdrawnfor analysis. The before-and-after analyses are as follows:

Alphn-Acetylenes 2-l3utyne Cyclopentadiene Before Na, After Na, BeforeNa, After Na, Before Na, After Na, Wt.pereent wt. per molpermolperwt.perocnt wt.percent cent cont cent This sodium treated isoprene ispassed downwardly 8 through a column packed with 305 lbs. of 5 Angstrommolecular sieves. A'total of 1,588 lbs. of isoprene containing 21 totalof 30.2 lbs. of Z-butyne produces a final percolate averaging 0.2 molpercent of Z-butyne. At this point the molecular sieves have absorbed10% of their weight of 2-butyne and obviously require regeneration.After regeneration of the sieves, the isoprene containing 0.2 molpercent of 2-butyne is again passed through the column. After the secondpass the Z-butyne content is well below 0.1%.

Example IV The procedure of Example III is repeated except that afterthe filtration step (following the sodium treatment) the isoprene(containing ca. 1.25 wt. percent 2-butyne) is transferred to adistillation vessel. Distillation is commenced under a 10:1 reflux ratiotaking off between 10 and 20% by volume of the charge. The pot residue(isoprene) is found to contain only 0.75 mol percent or 0.59 wt. percentof Z-butyne, this being a reduction of 53% in the 2-butyne content. Atthis point the apparent 2-butyne content has been reduced in two stages,for an over-all reduction of 60%. When such isoprene is passed throughthe sieve column of Example III, approximately 2.5 times the weight ofisoprene can be treated before the sieves require regeneration.

Example V The procedure of Example III is repeated except that the sievetreatment is carried out using a fixed bed column with the isoprenebeing passed up the column in the form of vapor. While this vapor styleof sieve treatment is slower (i.e. requires a larger column) than liquidphase adsorptions, the 2-butyne content is very elfectively removed.

I claim:

1. A process for preparing a polymerization grade isoprene containingless than 0.1%/Wt. of total polymerization retarding impurities from acrude isoprene derived from the cracking of petroleum and including, asimpurities, alpha-acetylenes and 2-butyne which comprises the stepwisetreatment, carried out in the order given below and under an inertatmosphere, of (1) mixing the said crude isoprene with an excess ofsodium metal in the form of colloidal particles from 1 to 10 microns indiameter, said excess of sodium metal being not greater than more thanthe amount required to react with the said alpha-acetylenes in saidcrude isoprene, (2) agitating the resulting mixture, at a temperaturebelow 40 C. until the said colloidal particles of sodium have reactedwith the said alpha-acetylenes, (3) filtering the said colloidal sodiumparticles from the remaining isoprene, and (4) contacting the resultingsodium-treated isoprene with molecular sieves having a channel diameterof 5 Angstrom units to produce isoprene containing less than about0.1%/wt. of total polymerization retarding impurities.

2. The process of claim 1 wherein the said sodiumtreated isoprene isdistilled under reflux to the extent of removing as overhead from about5 to about 20% of the total sodium-treated isoprene before the latter isbrought into contact with said molecular sieves.

References Cited in the file of this patent UNITED STATES PATENTS ISoday Apr. 23, 1946 OTHER REFERENCES

1. A PROCESS FOR PREPARING A POLYMERIZATION GRADE ISOPRENE CONTAININGLESS THAN 0.1%/WT. OF TOTAL POLYMERIZATION RETARDING IMPURITIES FROM ACRUDE ISOPRENE DERIVED FROM THE CRACKING OF PETROLEUM AND INCLUDING, ASIMPURITIES, ALPHA-ACETYLENES AND 2-BUTYNE WHICH COMPRISES THE STEPWISETREATMENT, CARRIED OUT IN THE ORDER GIVEN BELOW AND UNDER AN INERTATMOSPHERE, OF (1) MIXING THE SAID CRUDE ISOPRENE WITH AN EXCESS OFSODIUM METAL IN THE FORM OF COLLOIDAL PARTICLES FROM 1 TO 10 MICRONS INDIAMETER, SAID EXCESS OF SODIUM METAL BEING NOT GREATER THAN 75% MORETHAN THE AMOUNT REQUIRED TO REACT WITH THE SAID ALPHA-ACETYLENES IN SAIDCRUDE ISOPRENE, (2) AGITATING THE RESULTING MIXTURE, AT A TEMPERATUREBELOW 40* C. UNTIL THE SAID COLLOIDAL PARTICLES OF SODIUM HAVE REACTEDWITH THE SAID ALPHA-ACETYLENES, (3) FILTERING THE SAID COLLOIDAL SODIUMPARTICLES FROM THE REMAINING ISOPRENE, AND (4) CONTACTING THE RESULTINGSODIUM-TREATED ISOPRENE WITH MOLECULR SIEVES HAVING A CHANNEL DIAMETEROF 5 ANGSTROM UNITS TO PRODUCE ISOPRENE CONTAINING LESS THAN ABOUT0.1%/WT. OF TOTAL POLYMERIZATION RETARDING IMPURITIES.